The catalytic cracking process is widely used in the petroleum refinery industry for the conversion of relatively high boiling point petroleum feedstocks into lower boiling products, especially gasoline. In fact, the catalytic cracking process has become the preeminent process in the industry for this purpose. At present, the fluid catalytic cracking process (FCC) provides the greatest proportion of catalytic cracking capacity in the industry although the moving, gravitating bed process also known as Thermofor Catalytic Cracking (TCC) is also employed. The present invention is primarily applicable to FCC but it may also be employed with TCC.
The increasing necessity faced by the refining industry for processing heavier feedstocks containing higher concentrations of metal contaminants and sulfur presents a number of problems. Sulfur present in the feed tends to be deposited on the catalyst as a component of the coke which is formed during the cracking operation although most of the sulfur passes out of the reactor with the gaseous and liquid products from which it can later be separated by conventional techniques. It is, however, the sulfur containing coke deposits which form on the catalysts which are a particularly prolific source of problems. When the spent catalyst is oxidatively regenerated in the regenerator, the sulfur which is deposited on the catalyst together with the coke is oxidized and leaves the regenerator in the form of sulfur oxides (SO.sub.2 and SO.sub.3, generically referred to as SO.sub.x) together with other components of the flue gas from the regenerator. Because the emission of sulfur oxides is regarded as objectionable, considerable work has been directed to the reduction of sulfur oxide emissions from the regenerators of catalytic cracking units. One method for doing this employs a metal oxide catalyst additive which is capable of combining with the sulfur oxides in the regeneration zone so that when the circulating catalyst enters the reducing atmosphere of the cracking zone again, the sulfur compounds are released in reduced form so that they are carried out from the unit together with the cracking products from which they are subsequently separated for treatment in a conventional manner. The additive is regenerated in the cracking zone and after being returned to the regenerator is capable of combining with additional quantities of sulfur oxides released during the regeneration. U.S. Pat. No. 3,835,031 describes the use of Group II metal oxides for this purpose; U.S. Pat. No. 4,071,436 describes the use of a catalyst additive comprising separate particles of alumina which functions in a similar way and U.S. Pat. No. 4,071,416 proposes the addition of magnesia and chromia to the alumina containing particles for the same purpose. U.S. Pat. Nos. 4,153,534 and 4,153,535 disclose the use of various metal-containing catalyst additives which are stated to be capable of reducing sulfur oxide emissions with cracking catalyst containing CO oxidation promoters.
The use of magnesium aluminate spinels for the reduction of sulfur oxide emissions is described in U.S. Pat. Nos. 4,469,589 and 4,472,267. The spinel catalyst additive is effective in the presence of conventional CO oxidation promoters such as platinum and in addition, a minor amount of a rare earth metal oxide, preferably cerium, is associated with the spinel.
The presence of metal contaminants in FCC feeds presents another and potentially more serious problem because although sulfur can be converted to gaseous forms which can be readily handled in an FCCU, the metal contaminants generally tend to accumulate in the unit. The most common metal contaminants are nickel and vanadium which are generally present in the form of porphyrins or asphaltenes and during the cracking process they are deposited on the catalyst together with the coke formed during the cracking operation. Because both these metals exhibit dehydrogenation activity, their presence on the catalyst particles tends to promote dehydrogenation reactions during the cracking sequence and this results in increased amounts of coke and light gases at the expense of gasoline production. It has been shown that increased coke and hydrogen formation is due primarily to nickel deposited on the catalyst whereas vanadium also causes zeolite degradation and activity loss as reported in Oil and Gas Journal, 9 Apr. 1984, 102-111. See also Petroleum Refining, Technology and Economics, Second Edition, Gary, J. H. et al, Marcel Dekker, Inc., New York, 1984, pp. 106-107. The mechanism of vanadium poisoning of cracking catalysts is described in the article by Wormsbecker et al in J. Catalysis 100, 130- 137 (1986). Essentially, the vanadium compounds present in the feed become incorporated in the coke which is deposited on the cracking catalyst and in the regenerator is oxidized to vanadium pentoxide as the coke is burned off. The vanadium pentoxide is then posited to react with water vapor present in the regenerator to form vanadic acid which is capable or reacting with the zeolite catalyst, destroying its crystallinity and reducing its activity.
Because the compounds of vanadium and other metals cannot, in general, be readily removed from the cracking unit as volatile compounds, the usual approach has been to passivate them or render them innocuous under the conditions which are encountered during the cracking process. One passivation method has been to incorporate additives into the cracking catalyst or separate particles which combine with the metals and therefore act as "traps" or "sinks" so that the active zeolite component is protected. The metal contaminants are removed together with the catalyst withdrawn from the system during its normal operation and fresh metal trap is added together with makeup catalyst so as to effect a continuous withdrawal of the deleterious metal contaminants during operation. Depending upon the level of the harmful metals in the feed to the unit, the amount of additive may be varied relative to the makeup catalyst in order to achieve the desired degree of metals passivation. Additives proposed for passivating or trapping various metal poisons include antimony for controlling nickel poisoning, as discussed by Wormbecker op cit, and tin which has been used for processing various high metal feedstocks. Other additives proposed for controlling vanadium include the alkaline earth metal oxides, especially magnesium oxide and calcium oxide (Wormsbecker, op cit) as well as other alkaline earth metal and rare earth compounds e.g. lanthanum and cerium compounds, as described in U.S. Pat. Nos. 4,465,779; 4,519,897; 4,485,184; 4,549,958; 4,515,683; 4,469,588; 4,432,896; and 4,520,120. These materials which are typically in the oxide form at the temperatures encountered in the regenerator presumably exhibit a high reaction rate with vanadium to yield a stable, complex vanadate species which effectively binds the vanadium and prevents degradation of the active cracking component in the catalyst.
For economic reasons, if for no others, it would be advantageous to use a single additive which is effective for both metals and SO.sub.x removal. Unfortunately, however, there appears to be no correlation between activity as a metals passivator and activity as an SO.sub.x trap. For example, alumina which is effective as an SO.sub.x trap as described in U.S. Pat. No. 4,071,436, exhibits poor affinity to interact with vanadium and alkaline earth metal oxides have been reported to lose their activity for sulfur capture if subjected to repeated cycling (see U.S. Pat. No. 4,472,267). For this reason, it has generally been expected that it would be necessary to use two separate traps in order to handle cracking feeds containing high levels of metals as well as significant quantities of sulfur.